1. Field of the Invention
The present invention relates to a resin composition, which has a good processability and can provide a molded product having high flame-retardancy and excellent mechanical properties, and a flame retardant plastic optical fiber cable having a covering layer of such a flame retardant resin composition.
2. Description of the Related Art
A resin composition containing a halogen-containing flame retardant is known as a flame retardant resin composition. It is pointed out that during combustion this resin composition evolves a halogen-containing gas which is harmful to the human body.
In order to solve the above problem, a halogen-free flame retardant resin composition comprising an ethylene polymer and incorporated therein a metal hydroxide has been proposed, and this flame retardant resin composition has been used as a flame retardant covering material for electric wires and cables. However, the use of the halogen-free flame retardant covering material as a flame retardant covering material for plastic optical fiber cores gives rise to several problems. Specifically, the molding temperature of the flame retardant covering material is adjusted to the temperature at which the electric wire cable is formed. When a flame retardant plastic optical fibercable is molded under this condition using the above covering material, the plastic optical fiber core undergoes thermal damage, which results in a significant increase in light transmission loss in the optical fiber cable, so that the optical fiber cable cannot be put to practical use. Further, the flame retardant covering material has an excessive adhesion to the electric wire cable. This gives rise to the following problem when the covering material is applied to a plastic optical fiber cable. Specifically, in a plastic optical fiber cable prepared by using this covering material, when the ends of the plastic optical fiber cable are treated, the ends of the covering material (cabling material) should be peeled off from the core. In this case, it is very difficult to successfully peel the end portions from the core.
Regarding the flame retardant covering material for the formation of a flame retardant plastic optical fiber cable, Japanese Unexamined Patent Publication (Kokai) No. 2-257105 discloses an invention directed to a composition comprising an ethylene polymer, magnesium hydroxide and red phosphorus. Plastic optical fiber cables having excellent flame retardancy among the plastic optical cables using a composition according to the above invention have at least one of the problems: i) the adhesion between the flame retardant covering layer and the core is so high that it is difficult to peel off the covering material at the end of the optical fiber cable; ii) the ductility of the flame retardant covering layer of the cable is so low that the bendability is poor; and iii) the adhesion between the flame retardant covering layer and the plastic optical fiber core is so low that the interface of the covering layer and the core is likely to peel, that is, the handleability is unsatisfactory. Plastic optical fiber cables free from the above drawbacks and having a high flame retardancy have not been developed in the art.
Optical fibers are utilized in light transmission, illumination, decoration, design, display and other fields. Inorganic glass optical fibers having light transmitting properties good enough to transmit light in a wide range of wavelengths are known in the art. The inorganic glass optical fibers, however, have a problem in that they are likely to break, so that the bendability is poor. On the other hand, plastic optical fibers have excellent bendability and have become useful in various optical communication fields including short-distance LANs.
The plastic optical fiber cable comprises a single thin optical fiber or a bundle of a plurality of thin optical fibers and a protective covering material covering the periphery of the optical fiber(s). This plastic optical fiber cable has been utilized in the field of optical communication and communication within vehicles, such as automobiles, and as light guides for various types of equipment. When the optical fiber cables are used in a high-temperature atmosphere, they should have a high flame retardancy.
In the conventional flame retardant plastic optical fiber cable, a plasticized vinyl chloride resin is used as the covering material. The optical fiber cable using the plasticized vinyl chloride has problems of evolution of a halogen-containing gas during combustion and a deterioration in light transmitting properties due to damage to the plastic optical fiber core by the plasticizer. On the other hand, as described above, the plastic optical fiber cable using as the flame retardant the resin composition comprising magnesium hydroxide, red phosphorus and ethylene polymer as proposed in the above-mentioned Japanese Unexamined Patent Publication (Kokai) No. 2-257105 does not have well-balanced properties.